Recently, as liquid crystal display televisions increase in size, larger-size backlights are required. To meet the needs of growth in size as well as energy saving and price reduction, U-shaped or square-U-shaped bent-tube fluorescent lamps are produced by bending approximately center portions of long straight-tube fluorescent lamps to develop a backlight using these bent-tube fluorescent lamps as light sources instead of conventional straight-tube fluorescent lamps.
In the case of a backlight using the bent-tube fluorescent lamps, since relatively uniform temperature distribution is generated on screen less than 17-inch size, problems are less likely to occur although small differences are generated in temperature and luminance in the tube axis direction. However, as screens are further increased in size, temperature becomes higher in lamp electrode portions that are main heat generation sources as well as voltages applied to the lamps are increased due to longer lengths of the lamps, and leak currents tend to be generated near high pressure portions because of resultant stray capacitances between lamps and units. Because of increase in electric power for inverters due to increase in electric power for the lamps, the inverters located on the back sides of the lamp electrode portions generate more heat. As a result, temperature differences are increased in the lamp tube axis direction.
Since the leak current is generated, a lamp is lighted, near the high pressure portion in the tube axis direction, by a current value that is a sum of the rated current and the leak current, and this causes a luminance difference in the tube axis direction. The differences in temperature and luminance tend to increase in the tube axis direction due to larger sizes of the backlights as above, resulting in backlight luminance nonuniformity.
Therefore, a backlight has been proposed to apply light with uniform luminance even in the case of large-size liquid crystal displays (see, e.g., Patent Document 1). This proposition reduces a tube wall temperature difference in a lamp tube axis direction and a surface direction and reduces a luminance difference on a surface by alternatively arranging electrodes and bent portions of bent-tube fluorescent lamps and by disposing inverters immediately under respective reflection cases on the electrode feeding sides of the fluorescent lamps.
However, in the case of the invention described in Patent Document 1, although fluorescent tubes longer than at least the width of a screen are required, it is difficult to organize a sufficiently stable supplying system for a large screen size (e.g., 40-inch or more) because of the poor productivity in the bending process for the fluorescent tubes and the reduction in the manufacturing efficiency due to the longer lengths of the fluorescent tubes. The fluorescent tubes must be horizontally arranged because of preventing mercury, etc., within the fluorescent tubes from being unevenly distributed.
Therefore, recently, as described in Patent Document 2, a fluorescent tube drive mode in which two fluorescent tubes having lengths equivalent to screen width are connected in series and lighted is frequently employed, and the length may not be elongated more than necessary in this mode even in the case of fluorescent tubes mounted on backlights corresponding to large-size televisions. Large-size backlight devices may be produced making use of this advantage.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-268028
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-231034